1. Field of the Invention
Priority is claimed on Japanese Patent Application No. 2003-85759, filed Mar. 26, 2003, the content of which is incorporated herein by reference.
The present invention relates to a surface acoustic wave element, a frequency filter, an oscillator, an electronic circuit, and an electronic instrument.
2. Description of Related Art
In accompaniment with the remarkable developments in the field of communications centering on mobile communications such as mobile phones, recent years have seen a rapid increase in the demands placed on surface acoustic wave elements and various types of instruments that use surface acoustic wave elements. Surface acoustic wave elements were developed using single crystal such as quartz, however, in view of the movement in recent years towards still higher frequencies, or in view of the integration thereof with semiconductor devices, the development of surface acoustic wave elements formed by piezoelectric thin films is currently demanded.
Examples of surface acoustic wave elements that use piezoelectric thin films include zinc oxide thin films formed on a sapphire substrate (see, for example, Japanese Patent Application Unexamined Publication No. 7-50436), those formed by forming a diamond-like carbon film on a silicon substrate, and then forming a zinc oxide thin film on top of the diamond-like carbon film layer as a piezoelectric film (see, for example, Japanese Patent Application Unexamined Publication No. 1-03310), and those formed by forming a lithium niobate thin film on a sapphire substrate (see, for example, Appl. Phys. Lett. Vol. 62 (1993) pp. 3046–3048).
The integrating of a surface acoustic wave element such as those described above with a semiconductor device on a silicon substrate is useful for miniaturizing or improving the performance of various types of instrument that use surface acoustic wave elements. Examples of this include devices obtained by coupling a separately manufactured surface acoustic wave element formed by a single crystal with a semiconductor device formed on a silicon substrate (see, for example, Japanese Patent Application Unexamined Publication No. 6-120416). However, there are no examples of the integration of a semiconductor device with a surface acoustic wave element that uses a piezoelectric thin film.
The problems described below are inherent in the above described conventional technology.
Firstly, although it is possible to manufacture a high quality epitaxial thin film when forming a zinc oxide thin film or a lithium niobate thin film on a sapphire substrate, there has been a drawback in that sapphire substrates are more expensive than silicon substrates, and forming a semiconductor device such as a complimentary metal oxide semiconductor (CMOS) has been difficult.
On the other hand, it is difficult to epitaxially grow a zinc oxide thin film or a lithium niobate thin film on a silicon substrate. Moreover, even if it were possible to grow these directly so as to manufacture a surface acoustic wave element, because the silicon substrate is a semiconductor, the surface acoustic wave ends up leaking onto the silicon substrate so that there would be considerable insertion loss.
Furthermore, when growing a zinc oxide thin film, a lithium niobate thin film or a lithium tantalate thin film on a diamond-like carbon film formed on a silicon substrate, it has proven difficult to form a semiconductor device on the diamond-like carbon film.
In addition, when coupling a separately manufactured surface acoustic wave element formed by a single crystal on a silicon substrate on which a semiconductor device is formed, the characteristics of the surface acoustic wave element are determined by the cut angle of the material. In contrast, when forming a surface acoustic wave element using a piezoelectric thin film, it is possible to control the electromechanical coupling coefficient (represented below as k2) and acoustic velocity by controlling the film thickness of the piezoelectric thin film. As a result, there is a high degree of freedom when designing the element. Therefore, when forming a surface acoustic wave element from a single crystal, it might be considered that, if the single crystal is made thin, it may be possible to control the k2 and acoustic velocity characteristics in the same way as if the surface acoustic wave element were formed using a piezoelectric thin film. However, currently, it is substantially impossible to make the thickness of a single crystal thin enough to be at the level of a thin film.
The present invention was conceived in view of the above circumstances, and it is an object thereof to provide a surface acoustic wave element that has improved performance as a result of the film quality of the piezoelectric thin film being improved, and that, because it is possible to form a semiconductor device on a single crystal substrate, can be integrated with a semiconductor device, and that prevents leakages of the surface acoustic wave to the single crystal substrate. It is also an object of the present invention to provide a frequency filter, an oscillator, an electronic circuit, and an electronic instrument that are equipped with this surface acoustic wave element.